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Microsoft released the second installment of its now monthly security bulletins, patching three software holes in Windows systems that it said were "critical" security risks and a fourth problem with Microsoft Office that the company rated "important."

Taken together, the security holes could allow attackers to set up Web pages to take advantage of vulnerable systems and read files or run attack code on a remote user's Windows machine, Microsoft said.

One bulletin, MS03-049, affects Windows 2000 and Windows XP workstations and fixes a critical buffer overrun vulnerability in a Windows service called the Workstation Service, which manages requests for files or printing services on a local area network. That service is turned "on" by default in Windows and could be compromised by an attacker using an improperly formatted network message that was sent to a vulnerable machine.

A second bulletin, MS03-051, patches two critical problems that were discovered in Microsoft's FrontPage Server Extensions, which are installed by default with the Internet Information Services (IIS) on certain versions of Windows 2000 and allows technical staff to create, manage and add features to Web pages.

Microsoft said that it fixed a buffer overrun in the FrontPage Server Extensions that could enable an attacker to place and run attack code on a vulnerable machine, as well as a flaw in the way that a component called SmartHTML interpreter works that made it vulnerable to denial of service attacks.

The company also issued a new cumulative security patch, MS03-048, for the Internet Explorer (IE) Web browser. The new patch updates previous cumulative patches for IE and sews up five new vulnerabilities in the product, including a problem with the browser's cross domain security model, which governs how different Web sites share information and interact with the Windows operating system.

The fourth patch, MS03-050, fixes a security hole in some versions of Microsoft's Excel spreadsheet and Word word processing products that could enable an attacker to embed an attack in a small program known as a macro. Machines running vulnerable versions of those products could have malicious code placed and run on their Windows system after opening an infected spreadsheet or Word document, Microsoft said.

While all the security holes disclosed by Microsoft deserve prompt attention from network administrators, the Workstation Service and FrontPage Server Extensions vulnerabilities are particularly serious, according to Vincent Gullotto, vice president of McAfee AVERT at Network Associates.

The problem with the Workstation Service that Microsoft identifies in MS03-049 could be particularly troublesome for network administrators, because it affects every Windows 2000 and Windows XP machine, rather than less-plentiful servers, and because the vulnerability could allow attackers to launch their attack without any user interaction, Gullotto said.

MS03-051 is also dangerous, given the widespread use of Internet Information Server and the ability of attackers to remotely exploit the security hole and compromise vulnerable machines, he said.

Although Microsoft did not rate the Microsoft Office security hole critical, that too could be an attractive target for attackers, Gullotto said.

Attacks that use macros are less common than they used to be, he said. However a vulnerability such as the one disclosed in MS03-050 that allows remote code execution and the rich supply of systems running affected versions of Microsoft Excel and Microsoft Word could prompt hackers to write proof-of-concept code for the vulnerability that could then be used in attacks, Gullotto said.

Microsoft only last month switched to a system of releasing monthly security bulletins, replacing a system of weekly security updates.

The company made the change in response to complaints from Microsoft customers about the difficulty of staying on top of the weekly releases, Microsoft said.

Microsoft said it reserves the right to release bulletins at any time, however, and will do so when it feels customers are in imminent danger of being attacked because of a known software vulnerability.

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